Firefox Browser for Android's default settings requires you to scroll up somewhat rapidly on the page you're currently reading to reveal the browser's auto-hidden URL bar and menu. I tend to loathe this functionality because it means slightly losing my place every time I want to multitask when reading something.

I suppose it wouldn't be so bad as part of a website when viewed from a desktop environment, because you can easily highlight where you left off on the page much easier than you can on a tablet, for example.